Attempts to grow in vitro cells from canine lymphoid organs led to observation of the occurrence of a spontaneous, delayed and transitory growth of cells in suspension in two lymphoma cultures (out of 20) but also from one (out of 20) dog with unrelated disorder. These cells, with a lymphoblastoid cytological appearance, lack detectable immunoglobulins in cases studied originating from lymphomatous dogs, and have a pseudodiploid canine karyotype. Neither appearing fully immortalized nor showing any tumorigenicity in nude mice or immunosuppressed newborn rats, these may be of lymphoblastoid nature, rather than arising from some tumorous cells. The mechanisms that have triggered the cell growth are still unknown, but such cells could remind of human lymphoblastoid cells, associated with lymphotrophic viruses, still unknown in the dog.